Many applications require an amplifier to handle DC as well as AC input signals. In such applications, the DC input offset voltage and drift may introduce significant errors. Bipolar amplifiers have offsets on the order of millivolts and drifts on the order of microvolts/degree C., but have poor input impedance characteristics. FET amplifiers display better input impedance characteristics, but do not have satisfactory input offset voltages or drifts.
One previous solution to the drift problem was the use of a chopper amplifier. The major disadvantage of this technique is the bandwidth limitation. The upper limit of the bandwidth is determined by the "chopping" frequency, generally not more than a few hundred kilohertz.
Another previous solution was to combine a low pass amplifier in parallel with a high pass amplifier to handle input signals from DC to high frequency. One disadvantage of this arrangement is diminished performance near the crossover frequency of the two amplifiers. A second disadvantage is that close thermal matching of the transistors in the input stages of the two amplifiers is needed for satisfactory performance.
One application in which an amplifier with very low DC drift finds utility is the voltmeter. Most voltmeters require separate amplifiers for DC and AC signals because of the difficulty of achieving very low DC drift while maintaining adequate high frequency performance. A major savings in components would result if a single broadband amplifier could handle both DC and AC measurements.